The present invention relates to a novel system for controlling a vehicle antilock brake system (ABS) by means of stress sensors for detecting road surface friction forces of road surface friction coefficients for respective wheels which insures safe steering without locking of its wheels even during sudden braking.
There has been a mounting interest in the so-called antilock brake system which controls the brake fluid pressure on the wheel within a certain range (about 20%) of slip ratio between braking force and cornering force during sudden braking and an increasing number of vehicles are carrying such ABS systems for safe steering even during sudden braking.
The antilock brake system generally comprises a wheel speed sensor for detecting the locking tendency of the tire (wheel), a controller for outputting an actuator drive command according to the current wheel speed data and an actuator which, in response to said drive command, adjusts the brake fluid pressure. Among the known modes of ABS control are the three-system control (selectro-control) mode in which the brake fluid pressures to the front right and left wheels are independently controlled and, with one of the two rear wheels which is more liable to be locked as a reference, the brake fluid pressures for both rear wheels are controlled as a unit. A two-system control exists in which the front and rear wheels are respectively controlled as units or the diagonally located wheels are simultaneously controlled. Also, a simultaneous front and rear wheel control mode exists in which one of the rear wheels is controlled by the selectro-control method and with one of the front wheels which is harder to control as a reference, the brake fluid pressures to the two front wheels are simultaneously controlled.
Since the conventional antilock brake system described above uses one controller for controlling the brake fluid pressures to the four wheels either through three-system control or through two-system control, a long pipeline is required between the wheel cylinder of each wheel and the actuator. In the case of a large-sized vehicle such as a trailer or a large bus, which has a great overall length, the brake fluid pipeline has to span a great distance, with the result that not only a time lag is inevitable after the actuator receives a drive command and before the wheel cylinder of the wheel is supplied with a brake fluid pressure but also a transmission loss of the brake fluid pressure is liable to occur, so that the system cannot provide for exact brake control and, hence, cannot be said to be a fully safe antilock brake system.
The conventional ABS employing wheel speed sensors is a system which automatically controls the brake so as to bring the slip ratio into a certain range based on chassis speed and wheel speed but since the relationship between road surface friction coefficient and slip ratio is a variable dependent on the changing road surface condition, the conventional system may not provide for the maximal braking force depending on the road surface condition, with the result that the minimum braking distance cannot be insured for certain.
Furthermore, since the chassis speed is a value estimated from wheel speeds, the accuracy of slip ratio control is not high enough and in order to find the exact chassis speed, a complicated device such as a ground speed sensor or a chassis deceleration sensor is needed.